The Immune System

Question 1

Describe the main features of B lymphocytes.

Answer 1

Represent 20-30% of lymphocytes in the blood.
Produced and matured in the bone marrow.
Most reside in secondary lymphoid organs.

Question 2

Which immune response are B lymphocytes involved in?

Answer 2

Involved in immune response producing antibodies.

This type of immune response is called humoral immunity.

Question 3

What is an antibody (immunoglobulin)?

Answer 3

A large Y-shaped protein produced by plasma cells to neutralize pathogens.

Antigen binding sites recognize antigens on foreigh substances.

5 classes: igm, igd, igg, iga, ige.

Question 4

What is an antigen?

Answer 4

A toxin or other foreign substance that induces immune response, they can be molecules or part of foreign substances.

Different antibodies recognise different antigens on the same bacteria.

Question 5

What are B cell receptors?

Answer 5

Receptors on the surface of B lymphocytes.

Similar in shape to immunoglobulin but are located in the membrane. They are also a site for recognition of antigen.

B lymphocytes will produce antibodies after it has recognised a pathogen with a B cell receptor.

Receptor also has a complex that signals information into the cytoplasm.

Question 6

What happens once lymphocyte B recognises an antigen?

Answer 6

When a B cell encounters a foreign antigen, it binds to it with the receptor and becomes activated. The lymphocyte often needs help from lymphocyte T helper for this interaction.

Then the lymphocyte specific for this antigen will start proliferating, creating millions of identical cells.

They will either differentiate into cells producing and release antibodies (plasma cells), or stay as memory cells.

Question 7

What are the appearances of lymphocyte B vs T in a blood smear?

Answer 7

When lymphocytes B and T are both observed in a blood smear, they are not distinguishable.

Question 8

What distinguishable difference is there once lymphocyte B becomes plasma cells?

Answer 8

When lymphocyte B becomes plasma cells, the amount of RER increases a lot as it contributes to the formation and release of antibodies. So the cell becomes bigger with a very expanded ER.

Question 9

How do lymphocyte T helper cells activate lymphocyte B cells?

Answer 9

Lymphocyte T helper cells produce factors that will stimulate the activation of lymphocyte B to differentiate.

Question 10

What are plasma cells?

Answer 10

B cells activated by antigen.
Located in loose CT or lymphoid organs.

Produces millions of antibodies that destroy foreign substances bearing the antigen.

Question 11

Describe the features of T lymphocytes.

Answer 11

60-80% of lymphocytes in blood.

Produced in bone marrow but migrates to mature in the thymus.

Circulates continuously between blood, lymph and secondary lymphoid organs.

3 categories: helpers, cytotoxic and suppressors.

Question 12

What immune response are T lymphocytes involved in? Why?

Answer 12

Cell mediated immunity.

They have an active action on the immune system and participate in the elimination of cells.

Question 13

Describe features of the T cell receptor.

Answer 13

Does not recognize antigens that are soluble.

T helpers need APCs (Antigen Presenting Cells), they will have an MHC2 (major histocompatibility complex 2) on its surface, to recognize the antigen.

Each T cell can only recognize one type of antigen.

Question 14

How do T helper lymphocytes (CD4) function in immune response?

Answer 14

Activated by APCs (dendritic cells and macrophages) that digest foreign antigens and expose them together with MHC-II on the PM.

Proliferate and release interleukins and interferon gamma. This activates B cells, cytotoxic T lymphocytes, dendritic cells and macrophages.

Lymphocyte helpers need to be stopped, so in the final periods of activation, they become suppressor T lymphocytes to stop immune reaction.

Question 15

How do cytotoxin T lymphocytes (CD8) function in immune response?

Answer 15

Recognizes cells that have antigens exposed on their surface and kills these cells.

Releases cytotoxins which induce apoptosis in the infected cell as well as perforins which causes perforations in the cell membrane.

Granzymes are also released which will destroy infected cells.

Question 16

How do suppressor T lymphocytes function in immune response?

Answer 16

Derived from activated Helper T lymphocytes.

Produce molecules (IL-10, TGFbeta) that inhibits activity of other lymphocytes and terminates immune response.

Question 17

How do natural killer cells function in immune response?

Answer 17

Large cells in blood and lymphoid organs.

Have many lysosomes in the cytoplasm.

CD16 Fc receptors on the surface that recognize cell-bound antibodies. Then directly destroys the cell/pathogen with CD16 on the surface.

Can destroy tumour cells by triggering apoptosis.

Question 18

What are innate immune responses?

Answer 18

Physical barriers protect from infection, e.g. the skin, saliva, stomach acids..etc.

Components of our blood also contribute to our immune response, such as neutrophils, macrophages, basophils..etc.

Question 19

What are acquired immune responses?

Answer 19

Acquired immune response can be humoral immunity or cell mediated immunity.

Question 20

Describe lympho-hematopoietic tissues.

Answer 20

-

Question 21

Describe myeloid tissues.

Answer 21

Form the bone marrow.

2 types of bone marrow: red and yellow.

The red is most active during hematopoiesis and then becomes yellow due to an increase in adipocytes.

Question 22

Describe the general composition of bone marrow.

Answer 22

Made of reticular connective tissue.

Contains sinusoids - capillaries with a very leaky structure so that the cells from the bone marrow can easily go inside the circulation.

Hematopoietic and stromal cells.

Question 23

What is the thymus?

Answer 23

A primary lymphoid organ.

Has a lymphoid epithelial organisation made up of T lymphocytes + epithelial reticular cells.

Question 24

Describe the structure of Hassall’s corpuscles.

Answer 24

In the medulla, epithelial cells form Hassall’s corpuscles. They are characterized by a mass of epithelial cells that were thought to be dead cells. Now it seems that they contribute the Hassall’s corpuscular to the release of other substances in the promotion of lymphocyte T differentiation.

Question 25

Describe the structure of Hassall’s corpuscles.

Answer 25

In the medulla, epithelial cells form Hassall’s corpuscles. They are characterized by a mass of epithelial cells that were thought to be dead cells.

A network of epithelial cells will support lymphocyte T in between. The epithelial reticular cells promote differentiation of lymphocytes and produce substances similar to hormones.

Question 26

How do T lymphocytes mature in the thyroid?

Answer 26

The lymphocytes will arrive from the bone marrow at the level of the cortex, they start maturing and expose several factors on the surface. They will express both CD8 and CD4 at one point.

During maturation, none of these cells leave the thymus. If this does happen, the lymphocyte will recognize the antigen in our own cells as foreign and promote autoimmune response. Consequently there’s a blood barrier between the cortex and the medulla.

The lymphocytes maturing incorrectly need to be eliminated, this is partly carried out by macrophages.

Question 27

How do T lymphocytes mature in the thyroid?

Answer 27

The lymphocytes will arrive from the bone marrow at the level of the cortex, they start maturing and expose several factors on the surface. They will express both CD8 and CD4 at one point.

During maturation, none of these cells leave the thymus. If this does happen, the lymphocytes will recognize the antigen in our own cells as foreign and promote autoimmune response. Consequently there’s a blood barrier between the cortex and the medulla.

The lymphocytes maturing incorrectly need to be eliminated, this is partly carried out by macrophages.

Question 28

What are secondary lymphoid organs formed by?

Answer 28

Lymphoid organs are formed by lymphoid reticular tissue with rich vascularized tissue.

Question 29

Describe the general features of lymph nodes.

Answer 29

Small ovoid organs connected through lymphatic vessels.
Serve as filters to remove any pathogens.
Lymphocytes in the lymph nodes when in contact with foreign antigens will be activated.
The stroma is formed with reticular connective tissue, there are lymphocytes, antigen presenting sites, lymphatic vessels and blood vessels.

Question 30

Describe the general features of lymph nodes.

Answer 30

Small ovoid organs connected through lymphatic vessels.

Lymphocytes in the lymph nodes when in contact with foreign antigens will be activated.

The stroma is formed with reticular connective tissue, there are lymphocytes, antigen presenting sites, lymphatic vessels and blood vessels.

Question 31

Describe the structure of lymph nodes.

Answer 31

In the lymph node, a structure of the cortex and the medulla can be distinguished.

Circles can be seen called germinal centres in the superficial cortex.

Question 32

How can lymph nodes by stained?

Answer 32

Reticular fibers are visible with silver staining.

Question 33

Describe the path taken by lymph through a lymph node.

Answer 33

In the lymph node, the lymph arrive on the external side through afferent vessels. They will go through the septum and exit from the hilum through efferent vessels.

When the lymph enters, it will be checked for pathogens by all the lymphocytes present in the area.

Question 34

Describe the circulation of blood through a lymph node.

Answer 34

Blood will go out at the level of the deep cortex and move towards the superficial cortex where there are the germinal centres. Then the blood will circulate again.

Question 35

Describe the action of lymphocytes in the lymph node.

Answer 35

The germinal centre represents a mass of B lymphocytes and follicular dendritic cells. They increase in size with the proliferation of B lymphocytes.

When a pathogen has entered the lymph node, the B lymphocytes are active and start proliferating. There’s a lighter centre which is seen when there’s an initial proliferation of B lymphocytes, with a darker outer mantle observed.

Amongst the nodules are the T lymphocytes.

Question 36

What is in the medulla of a lymph node?

Answer 36

Contains mainly blood vessels, large lymphatic sinuses, T lymphocytes and plasma cells organized in the medullary cords.

T lymphocytes and plasma cells will be more abundant.